A semiconductor device having a movable portion provides, for example, a physical quantity sensor. In the sensor, the movable portion is formed on a semiconductor substrate. The movable portion is movable in accordance with a physical quantity such as acceleration applied to the sensor. The sensor further includes a fixed portion. The fixed portion and the movable portion form a capacitor having an electrostatic capacitance, which is changeable in accordance with the displacement of the movable portion so that the physical quantity applied to the sensor is detected.
A method for forming a movable portion is disclosed in U.S. Pat. No. 6,399,516 and No. 6,365,056. In this method, the movable portion is formed by using a notching effect. Specifically, a semiconductor layer on an insulation layer in a substrate is over-etched in a reactive ion etching process so that the semiconductor layer is separated from the insulation layer. Thus, the movable portion is formed. In this case, a trench forming process for forming a trench on the semiconductor layer and a separation process for separating the semiconductor layer from the insulation layer are successively performed under the same etching condition. Therefore, control (i.e., etching condition) for controlling the separation process is merely provided by a control of an etching time. Thus, a form of a notch, i.e., a shape of the movable portion is not controlled precisely. Specifically, a gap between the movable portion and the insulation layer is not controlled.
Thus, the sensor has different movable portions having different gaps. However, it is preferred that the gap between the movable portion and the insulation layer is uniformed to obtain uniform sensor characteristics.
Further, in the separation process, a needle like protrusion may be formed on the inner wall of the trench, i.e., on the inner wall of the notch, which faces the insulation layer. The protrusion of the notch may be broken when a large impact is applied to the protrusion so that the protrusion crashes the insulation layer. The broken protrusion may cause a particle, and the particle may cause a problem, for example, short circuit when the sensor is operating.
Furthermore, when the etching condition of the trench etching process is optimized so that the trench is etched appropriately in a depth direction and a sidewall of the trench is strongly protected by a protection film. In this case, in the separation process for etching the sidewall of the trench to form the movable portion, the etching rate of the sidewall of the trench in the horizontal direction becomes smaller; and therefore, the etching time for separating the semiconductor layer from the insulation layer, i.e., the process time becomes longer. Thus, the manufacturing cost becomes larger.